1. Field of the Invention
The present invention relates to an image supporting member used in copying machines, printers and the like. More specifically, the invention relates to improvements of an image supporting member effective for forming a color image by electrophotography or the like, a method of using the same and an image forming apparatus using the same.
2. Description of the Related Art
With respect to ordinary modes employing, for example, electrophotography, as this type of the image forming apparatus, the following image forming steps are conducted in forming a color image.
That is, illumination is applied to an original, and the reflected light is subjected to color separation with a color scanner. The colors are image-processed with an image processing device, and subjected to color compensation. Image signals of the resulting plural colors are converted into, for example, laser beams modulated with a semiconductor laser or the like according to the respective colors. The laser beams are applied to an image carrier made of an inorganic photoreceptor of Se, amorphous silicon or the like or an organic photoreceptor using a phthalocyanine pigment, a bisazo pigment or the like as a charge generating layer plural times one by one according to the respective colors to form plural electrostatic latent images. These plural electrostatic latent images are developed in order with charged toners of four colors, Y (yellow), M (magenta), C (cyan) and K (black). The developed toner images are transferred onto an image supporting member such as a paper from the image carrier made of the inorganic or organic photoreceptor, and fixed with a fixing device of a heat-pressing and fixing system. In this manner, the color images are formed on the image supporting member.
The color toner includes particles which have an average size of from 1 to 15 μm and are obtained by dispersing a colorant to a binder resin such as a polyester resin, a styrene/acrylic copolymer or a styrene/butadiene copolymer, and a fine particulate additive having an average particle size of from 5 to 100 nm and made of an inorganic material such as silicon oxide, titanium oxide or aluminum oxide or a finely divided resin of PMMA or PVDF wherein the additive is adhered to the surface of the particles.
With respect to the colorant, examples of Y (yellow) include Benzidine Yellow, Quinone Yellow and Hansa Yellow, examples of M (magenta) include Rhodamine B, Rose Bengale and Pigment Red, examples of C (cyan) include Phthalocyanine Blue, Aniline Blue and Pigment Blue, and examples of K (black) include carbon black, Aniline Black and a blend of color pigments.
As the image supporting member, a plain paper using a pulp raw material as a main component, a coat paper obtained by coating a mixture of a resin, a white pigment and the like on a plain paper, and a white film obtained by mixing a resin such as a polyester with a white pigment are used.
For example, as described in JP-A-2000-010329, JP-A-2000-003060 and JP-A-2002-091212, it is known that when forming an image with a high gloss equal to a gloss of a silver halide photographic print, an image supporting member obtained by using a plain paper, a coat paper or a white film as a base and forming thereon a layer made of a thermoplastic resin and having a predetermined thickness is preferable.
In case of a photographic print image, a thick image supporting member is generally preferable.
With respect to the transferring step, a method is known in which, for example, a transfer roll or a transfer belt made of a dielectric material or the like is previously mounted opposite to an image carrier made of a photosensitive material or the like, an image supporting member is previously attached onto the transfer roll or the transfer belt, and the transfer roll is biased or a predetermined transfer member (for example, a transfer corotron, a biased transfer roll or a biased transfer brush) is mounted on a back surface of the transfer belt, whereby an electric field having an opposite polarity to that of the charging of the toner is applied from the transfer roll or the back surface of the transfer belt to electrostatically transfer toner images on the image supporting member one by one according to the respective colors.
In the transferring step, another method is known in which, for example, a belt-shaped intermediate transfer member made of a dielectric material is mounted opposite to an image carrier made of a photosensitive material or the like, an electric field having an opposite polarity to that of the charging of the toner is applied from the back surface of the intermediate transfer member with a predetermined primary transfer member (for example, a transfer corotron, a biased transfer roll or a biased transfer brush) to transfer the toner images formed on the image carrier onto the intermediate transfer member one by one according to the respective colors and once form color toner images on the intermediate transfer member, and an electric field having an opposite polarity to that of the charging of the toner is then applied from the back surface of a substrate with a predetermined secondary transfer member (for example, a transfer corotron, a biased transfer roll or a biased transfer brush) to electrostatically transfer the color toner images onto the substrate.
In the fixing step, there is known a heat-pressing and fixing method in which heat sources such as incandescent lamps are embedded in a pair of fixing rolls which are urged against each other, and an image supporting member with color toner images transferred is passed through the pair of fixing rolls to heat-melt the color toners and fix them on the image supporting member, or a cooling-separation-fixing method in which a fixing belt with a release layer of a silicone rubber or the like formed on the surface is hanged on plural tension rolls, a pair of fixing rolls are mounted opposite to each other with the fixing belt therebetween, heat sources such as incandescent lamps are embedded in the fixing rolls, the image supporting member is passed through the pair of fixing rolls while the fixing belt is overlaid on the image supporting member with the color toner images transferred to heat-press and fix the toner images, and the fixing belt and the color toner images are separated after cooling the toner images to fix the color toner images on a substrate.
Especially when an image having a high gloss equal to a gloss of a silver halide photographic print is formed, it is known that the latter fixing method is preferable. Further, a high gloss is uniformly provided regardless of an image density, by combining the latter fixing method with the foregoing substrate having the thermoplastic resin.
This type of the image forming apparatus in which the image supporting member having the thermoplastic resin layer is used as a base gives rise to technical problems that when a white PET film or coat paper is employed, the image quality is good, but the image supporting member is quite costly, whereas when a less costly plain paper is employed, no good image quality is obtained.
When the main component of the thermoplastic resin layer is an amorphous resin such as a polyester resin, a polystyrene resin or a polyacrylic resin, such a technical problem arises that all of a low-temperature fixability, a heat resistance, a mechanical strength are not satisfied.
That is, when reduction of an amount of energy consumption in the image formation is taken into consideration, a low-temperature fixability is an indispensable subject. For satisfying the low-temperature fixability, it is effective to decrease a molecular weight of a resin and decrease a glass transition temperature.
Meanwhile, when, for example, photographs having images with a smooth surface are stored in automobiles or warehouses in summer or left in a high-temperature atmosphere in transportation on a ship bottom or the like while image top surfaces and back surfaces, image top surfaces, or image top surfaces and album materials are overlaid, a problem of blocking (photographs are adhered without separation, or separated but damaged in top surfaces) might occur.
In this case, for improving a durability at a high temperature, namely, a heat resistance, it is effective to increase a glass transition temperature and increase a molecular weight.
A toughness in folding the image, namely, improvement in mechanical strength is also an important subject. For increasing the mechanical strength, it is effective to increase a molecular weight.
Thus, the improvement in mechanical strength and heat resistance is contrary to the improvement in low-temperature fixability. Especially, in case of obtaining an image having a high gloss equal to that of a silver halide photograph, a fixing temperature has to be increased. Accordingly, it is more difficult to satisfy all of the three requirements.
Thus, the present inventors have tried to use an image supporting member in which a light diffusion layer (corresponding to a light scattering layer) obtained by dispersing 30% by weight of a white pigment such as titanium oxide in a polyolefin resin such as polyethylene is formed on a raw paper made of at least a pulp raw material and having a basis weight of from 150 to 200 g/m2.
This image supporting member can be produced at relatively low cost and provide an image having a high whiteness.
In this case, however, there is a technical problem that a uniform good surface structure with a high gloss is not obtained throughout the image surface in the fixing step because a melt viscosity of the light diffusion layer on the surface of the image supporting member is high.